The invention relates to vehicle occupant restraint systems.
In restraint systems the gas bag is initially fully inflated with minimum delay. In order to enable the gas bag to absorb as much kinetic energy as possible, a valve is provided through which the gas can escape in a controlled manner. For this purpose the valve needs to have a negative coefficient characteristic, i.e. the heavier the vehicle occupant the smaller the exit flow of gas per unit of time is to be, so that the heavy vehicle occupant can be restrained with a greater force, whilst in the case of a lightweight vehicle occupant needing to be cushioned with less force the gas bag is required to become xe2x80x9csoftxe2x80x9d more quickly. Various types of restraint systems incorporating such a valve are known, the gas bag mostly being provided with a through-flow opening which becomes smaller as a function of the internal pressure. Thus EP 0 917 994 teaches a gas bag having a through-flow opening surrounded by a loop. On an increase in the internal pressure the gas bag expands, the loop is tightened and reduces the flow cross-section of the through-flow opening. Another variant is disclosed by DE 196 33 883 A1. In this case the through-flow opening is covered by a fabric part of reduced expandability so that when the gas bag fabric is highly tensioned the through-flow opening is closed. However, the static pressure needs to attain a minimum value before these valves will close, so in the initial phase of inflation a gas flow can exit which will then be no longer available for filling the gas bag, thus prolonging the inflation time. A somewhat different solution reads from EP 0 917 995 in which the through-flow opening in the gas bag is covered by a fabric part which billows when the gas is passing by, puckering its edge thus reducing the flow cross-section of the through-flow opening. Here too, a prior gas flow is needed which will then be lost for inflation.
The invention has the object of improving a vehicle occupant restraint system of the aforementioned kind such that the gas flow made available by the inflator is optimally utilizable for inflating the gas bag.
The invention provides a vehicle occupant restraint system comprising a compressed gas source, a gas bag inflatable with compressed gas furnished by the compressed gas source, and a valve through which the gas is able to escape from the restraint system, the gas source generating a gas flow, the valve being arranged such that it is impacted by said gas flow and the valve reducing its flow cross-section upon an increase in pressure of the gas flow. Since the compressed gas source furnishes the maximum flow pressure on ignition, the valve has, shortly after activation, its minimum flow cross-section because it is impacted by the gas flow, so that at least on commencement of the inflation action hardly any gas is lost. This thus ensures quickest possible inflation of the gas bag.
In one embodiment of the invention the characteristic of the valve is adjustable thus enabling, simply by setting the valve, compressed gas sources of the same type to be put to use for differing gas bags. In addition, it can be set how strongly the gas bag is to be inflated, for instance as a function of the position or weight of the vehicle occupant when a corresponding sensing device is provided.
In accordance with the preferred embodiment of the invention the compressed gas source is an inflator and the valve is integrated in the inflator. This, for one thing, simplifies fabricating the gas bag, since it now no longer needs to be provided with relief openings, and, for another, this minimizes the flow path between the inflator and the valve so that the valve is able to react as quickly as possible.
Advantageous aspects of the invention read from the sub-claims.